The immediate objective of this proposal is to describe the particle of tobacco mosaic virus (TMV) and other helical viruses, including tobacco rattle virus (TRV), in molecular detail. This forms part of a broader objective: to study the process of macromolecular assembly; in particular, the assembly of macromolecular structures involving proteins and nucleic acids. Assembly of sub-cellular structures is one of the principal steps in growth and differentiation, and control of assembly is thus a major regulatory mechanism. Tobacco mosaic virus is the ideal system for such studies, since its biology and physical chemistry are already extensively studied, and it offers the opportunity of relating its mechanism of assembly, and in particular the control of that assembly to its molecular structure. X-ray diffraction provides an excellent method of obtaining an image of the virus in sufficient detail to describe the moleculr structure and interactions. The model built from the 3.6 angstroms resolution map of TMV will be refined, and resolution extended at least to 3.0 angstroms. Structures of several strains of TMV will be determined, including the U2 strain (for which preliminary results are already available). Structures of the helical aggregates of TMV protein and a lead derivative of TMV will also be studied further. All of these systems have been chosen because of their importance in studying the carboxyl groups which form the control site in the assembly of TMV. The models will be compared with structures of other aggregates, principally the "disk" precursor to the virus, in order to visualize the molecular details of switching between the different states of aggregation of protein. The same methods will be applied to TRV, which is quite different from TMV in gross structure. A low-resolution map (ca. 6 angstroms) is aimed at initially, but eventually we hope to determine this structure in comparable detail to TMV. Since TMV is the only protein-nucleic acid interaction which has been described in molecular detail until now, it is important to examine another system to establish the generality or otherwise of our findings.